The mouse macrophage cell line P388D1, in conjunction with a superinduction protocol has been used to obtain relatively high levels of the macrophage immunostimulatory peptide, interleukin 1 (previously termed lymphocyte activation factor). The interleukin 1 has been purified to homogeneity with a yield of approximately 60 micrograms/5 l culture fluid. The purified interleukin 1 is composed of three charge species of identical molecular weight. Biologic studies have revealed that interleukin 1 may play a critical role in T cell activation as well as in rheumatoid synovial cell activation. Thus, via a single mediator, the macrophage may regulate the behavior of a number of cell types involved in immune and inflammatory responses.